In the oil industry, there are presently no known exploration techniques that can determine accurately how much oil is left behind, and if it is worth going after it. Moreover, often remaining oil is found in scattered, isolated droplets and patches. Oil companies are searching for more advanced and efficient ways to optimize the development of oil and gas fields. Increasing discovery of oil resources as well as increasing recovery in the major producing reservoirs has thus become a significant goal.
To get a closer look at the interior of a well, imaging technologies utilizing sensors that are injected in the well have been proposed. The present inventors have recognized, however, that the material composition of the imaging agent is an important key to the success of this technology. In one standard technology, for instance, developers utilize fluorescent molecule based tags (˜1 nm dimension) as a sensor or marker. Particularly, molecular species are injected on one side of the well and recovered at another side. These markers, however, are able to pass through the finest pores in the rock, and thus the technology suffers severely from slow diffusion and exiting. Moreover, molecular species currently used are not chemically stable under the harsh environmental conditions in a mixture of water and oil in the well (e.g., salinity between 10-20%, ph between 7.5 and 8.4), which drastically reduces the survival rate of the tags in the well. The problem is also compounded by the fact that an oil field usually is made up of a large number of sub-fields and wells that are isolated from one another (e.g., at a spacing of ˜1 km). Thus, tracers or imaging agents used in large system applications such as underground water or oil reserves suffer from a number of problems, and accordingly the sensing methodology suffers from a number of problems, including: low throughput due to the huge volumes involved and/or complicated porous structures involved, and low optical sensitivity. In the case of polymer-based tracer nanoparticles, there is low optical sensitivity, as the sensitivity of detection or counting of polymer-based particles by optical methods is low because the quantum efficiency of fluorescence/luminescence of polymer is extremely low. There is also poor selectivity, as selective detection is hampered by the fact that the emission of the polymer nanoparticles in the visible overlaps with that of hydrocarbon constituent of the oil.